1. Field of the Invention
The present invention relates to an apparatus and a method for correcting a positional shift containing in a positional-information coordinate output in a matrix form derived from a gamma camera apparatus, e.g., a single photon emission computerized tomographic (SPECT) apparatus.
2. Description of the Related Art
An overall construction of a typical gamma camera apparatus is represented in FIGS. 1 and 2. This gamma camera apparatus is constructed of a couch unit 1 which includes a couch 3 for supporting a biological body 2 under medical examination, e.g., a patient. The couch 3 is movable both a vertical direction Z (height) and a horizontal direction X by way of, for example, a height adjusting mechanism 4. A head portion of the biological body 2 is fixed by a headrest 5, and the biological body 2 into which a predetermined medicine containing a radio isotope (RI) has been injected lies on the couch 3. The gamma camera apparatus further includes a gantry unit 6 having a dome 7 for guiding the biological body 2. Around this dome 7, three sets of detectors (namely gamma cameras) 8A to 8C rotatably provided to detect gamma rays emitted from the radio isotope injected into the biological body 2. The gamma-ray detectors 8A to 8C are arranged, as shown in FIG. 3, in such a manner that these detectors 8A to 8C are equidistantly separated by 120.degree. with each other. It should be noted that an effective field for detecting gamma rays is defined by a region surrounded by a diameter "R" and a depth "l" within this dome 7.
While each of detectors 8A, 8B and 8C is rotated by 120.degree. around the biological body 2 with the above-described arrangement, since the gamma rays may be detected by these detectors 8A to 8C over 360.degree. range, a tomographic image of a desired portion within the biological body 2 under medical examination may be obtained based upon the detected gamma-ray data. The gamma camera apparatus having such an arrangement is known as a SPECT (single photon emission computerized tomography) apparatus.
Generally speaking, in the SPECT apparatus with the above-described three sets of gamma-ray detectors, three detectors 8A to 8C are rotated around a "common" rotation center"P". Assuming now that a perpendicular line "L" passes from this common rotation center "P" to an incident surface of each detector 8A to 8C, a point of intersection "P.sub.l " between the perpendicular line "L" and the incident surface of the detector is referred to as a geometrical center". In FIG. 3, reference numerals 9A to 9C indicate collimators.
On the other hand, a projection image of a biological body under medical examination medically measured by a gamma camera is obtained as numerical data with respect to each pixel of a matrix, i.e., an output signal matrix of a gamma-ray detector. FIG. 4 represents an output signal matrix of a gamma-ray detector (i.e., gamma camera) by which a relationship between an incident position of a gamma ray onto the detector 8 (8A, 8B or 8C), and a center position "P.sub.2 " of an output signal matrix of the detector 8 may be understood. Also, another relationship between an output from the detector 8 and a gamma-ray incident position onto the detector 8 has been corrected with having a linearity, as shown in FIG. 5. It should be understood that the above-described center position "P.sub.2 " on the output signal matrix of the detector 8 must be coincident with the geometrical center position "P.sub.1 " of the detector 8 with respect to the positional information in order to obtain a correct diagnostic tomographic image of a biological body.
FIGS. 6A and 6B illustrate the reason why the matrix's center position "P.sub.2 " must be coincident with the geometrical center position "P.sub.1 ". It is assumed that as shown in FIG. 6A, a point-shaped RI (radio isotope) source 11 is positioned at a rotation center "P" of the detector 8, and a positional shift "P" is present between the geometrical center position "P.sub.1 " and the center position "P.sub.2 " detected as the output signal of the detector 8. Projection data of the point-shaped RI source 11 appears on a gamma-ray detector. When this projection data is back-projected onto a matrix (i.e., output coordinate signals from the detector 8) so as to obtain a tomographic image from this projection data, the projection data of the point-shaped RI source 11 is back-projected onto another position which is positionally shifted by ".DELTA.P" from the geometrical center position "P.sub.1 " as shown in FIG. 6B. As a consequence, a ring-shaped artifact may be formed on a reconstructed image, which requires an offset correction with respect to the center position "P.sub.2 " on the output signal matrix of the detector 8.
As previously described, since the geometrical center position "P.sub.1 " of the respective gamma-ray detector is not always coincident with the matrix's center position "P.sub.2 " thereof in the conventional SPECT apparatus, there is problem that a ring-shaped artifact may happen to occur in the resultant tomographic image. To the contrary, it will be most probably conceived that the conventional SPECT apparatus could be assembled by performing proper fine adjustments on these center positions "P.sub.1 " and "P.sub.2 ". However, there is another difficulty that highly positional alignment is necessarily required so as to achieve the above-described center coincidence.
The present invention has been made in an attempt to solve the above-described problems and has an object to provide an apparatus and a method for performing an offset correction in order to make coincident between a geometrical center position of a gamma-ray detector and a center position of an output signal matrix thereof in a gamma camera apparatus.